Pulp is generally produced in a refiner which comprises two opposed refining disks, at least one of which is rotary. The lignocellulose-containing material is supplied centrally through one of the disks, and is then disintegrated in the refining gap between the disks, in the presence of water. The refining disks are provided with a plurality of exchangeable refining segments in the shape of a sector of a circle, and they are formed with a refining surface, which is provided with elevations in the form of bars and intermediate grooves. The bars generally extend across substantially the entire refining surface. The direction of the bars can be radial or oblique in relation to the radius.
The fiber material is initially defibered in the refining gap between the refining surfaces; i.e., the fibers are separated. This takes place in the inner portion of the refining gap, where the distance between the refining surfaces is the greatest. As one then proceeds outwardly, the refining gap decreases in size, so as to effectuate the desired processing of the fiber material. To achieve this processing, great amounts of energy are required. Simultaneously, great amounts of steam are generated from the water which is carried with the fiber material.
Depending on the degree of processing which is desired, and thereby on the pulp quality, the refining surfaces can be designed in various ways. Pulp quality is also affected by other factors, such as the size of the refining gap, the liquid content in the fiber material, the feed temperature, etc.
The appearance of the refining surface is of great importance, particularly with regard to the length of the fibers in the processed fiber material. When the bars are oriented substantially radially on the refining surface, a large proportion of long and well-fibrillated fibers are obtained in the pulp. This can be explained by the fact that the fiber material in the refining gap orients itself with the fiber direction substantially parallel with the edges of the bars. The defibering and processing of the fiber material then takes place in such a way that the fiber material is substantially rolled between the bars on opposed refining surfaces, whereby the fibers are separated and fibrillated along their entire length. This type of pulp has high strength, and is thus particularly valuable in many connections, e.g., for newsprint. The energy consumption during production of this type of pulp is relatively high.
When the bars are oriented obliquely in relation to the radius of the refining segment, the proportion of long fibers in the pulp decreases, because the edges of the bars in this case have a cutting effect on the fiber material. At the same time, the fibrillation effect decreases as this cutting effect increases. This type of pulp has considerably lower strength properties, but is particularly suitable for the making of finer paper qualities where formation, printability and opacity are highly valued.
The angle of these bars with respect to the radius is also important for the feed of the material through the refining gap. When the bars are angled obliquely outward, as viewed backward from the direction of rotation thereof, an outward pumping action is obtained, while angling in the opposite direction has a braking effect. The stay time of the material in the refining gap is thus affected by the angle of the bars.
Known refining segments are generally thus designed so as to yield the desired properties of the pulp. This often requires one to make compromises with regard to the design of the refining surfaces in order to obtain a suitable balance between fibrillation and cutting of the fibers and, respectively, between feeding and braking.
It is therefore an object of the present invention to develop a refining segment which can be designed in order to yield optimum pulp while at the same time minimizing the energy consumption therewith.